This invention relates generally to methods and apparatus for testing core samples extracted from an oil or gas well. In particular, the invention relates to a method of determining elastic properties of a core sample and an apparatus useful in performing the method, which apparatus is for detecting changes in longitudinal and lateral dimensions and/or acoustic properties of the core sample.
A commonly utilized technique for stimulating the production of hydrocarbons from a subterranean rock formation penetrated by a well bore is to create and extend fractures in the formation. Generally, the fractures are created by applying hydraulic pressure on the formation from the well bore. That is, a fracturing fluid is pumped through the well bore and into the formation at a rate and pressure such that the resultant hydraulic force exerted on the formation causes one or more fractures to be created. The fractures are extended by continued pumping; and the fractures can be propped open or flow channels can be etched in the faces of the fractures with acid, or both can be done, to provide openings in the formation through which hydrocarbons readily flow to the well bore. Fracturing is also utilized in carrying out enhanced production procedures in subterranean formations (e.g., water flooding from an injection well to a production well) as well as in other applications.
In designing fracturing treatments to be carried out in subterranean rock formations, it is often necessary and always desirable to know the direction in which fractures will extend in the formation. Such knowledge enables more efficient reservoir management. For example, knowing such directional information allows one to better place production wells for maximizing production from the reservoir of hydrocarbons in the subterranean formation and to better place waterflood injection wells for increasing waterflood sweep efficiency by avoiding an injection well arrangement that would cause premature breakthrough of the injected fluid into the producing well.
Information that can be used to help predict the size of the fracture includes Young's modulus and Poisson's ratio, which describe elastic properties of rock. These can be determined by testing rock core samples that have been extracted from an oil or gas well in a known manner.
Young's modulus can be defined as the ratio of normal stress to the resulting strain in the direction of the applied stress. Stress can be applied to a core sample with a longitudinal or axial compressive force from a known type of press. The resulting longitudinal or axial strain is the yield or deflection measurable as the change in the longitudinal or axial dimension of the core sample.
Poisson's ratio can be defined as the ratio of lateral or radial strain to the longitudinal or axial strain for normal stress within the elastic limit. This is measurable using the aforementioned detected dimensional change in conjunction with a lateral or radial dimensional change detected in response to the applied stress.
Prior proposals to obtain the foregoing information include the method and apparatus disclosed in U.S. Pat. No. 5,325,723 to Meadows et al., which is incorporated herein by reference. A modification of this apparatus has been built and used. This modified apparatus is shown in FIGS. 1-3 of the present specification. Features of the modified apparatus beyond what is described in the 5,325,723 patent include: two diametrically opposed sensors 500 (FIG. 2) for measuring longitudinal deflection, which sensors are connected to brackets attached to end caps of the apparatus; two sensors 502 (FIG. 3) in a uniform spatial pattern in a single plane around the circumference of the core sample 504 to measure lateral deflection; a pressure sealed container 506 for the moving components of each lateral displacement sensor, which container is capable of withstanding design pressure of the entire test apparatus thus eliminating friction on the measurement rods of the lateral displacement sensors; attachment of the lateral deflection sensors by the pressure sealed containers directly to the support body 508 of the housing containing the core sample; physical dimensions defined to accommodate testing softer formation materials; loading spacers 510 adjacent the end caps to allow application of hydraulic pressure to the end of the sample to obtain true hydrostatic stress load application on all surfaces when used with simpler loading frames; a port in one of the loading spacers to allow fluid flow through the sample for measurement of physical flow properties (permeability) as the stress applied to the sample is changed, thereby allowing a determination of the effect of stress loading on the sample; the length of annular washers 512 adjacent ends of the core sample sealing sleeve being made sufficient to allow the use of O-rings on the washers and end caps, and on any spacers adjacent the end caps, to seal the flow path through the core sample; and spring pressure applied at the back end of the radial measurement rod of each lateral deflection sensor 502, which spring pressure is applied completely within the pressurized environment to ensure that the pad surface is in contact, with the outside of the sealing membrane, thereby eliminating the problem of isolation between the core sample and the pressurizing fluid which can be caused by a leaking perforation required to place inserts through the flexible membrane as described in the 5,325,723 patent.
Despite the disclosure in U.S. Pat. No. 5,325,723 and the apparatus shown in FIGS. 1-3 of the present specification, there is still the need for an improved core sample test method and apparatus. Specifically, there is the need for greater understanding of directional variation of mechanical properties in underground formations. To achieve this, there is the need for the capability to monitor lateral deformation in two orthogonal directions. This will enable observation of directional anisotropy in the core sample specimen. In the absence of directional anisotropy, this will enable an improved accuracy to be obtained in the magnitude of lateral deformation through averaging the diametrical changes. There is also the need for ensuring that no torsion forces are induced upon the sample from the sealing sleeve so that proper sealing and measurements can be obtained. There is further need in the industry for greater understanding of the relationship between dynamically derived mechanical properties for underground formations and laboratory measured static mechanical property measurements. Incorporation of acoustic travel time measurement using an acoustic transmitter and an acoustic receiver will allow correlation of travel time with stress magnitude in the laboratory test cell. This will allow the development of correlations between static and dynamic mechanical property data.